Variability among autumn migration patterns of Mongolian Common Shelducks (Tadorna tadorna)

The power of flight enables birds to exploit temporally constrained food resource abundance in habitats separated by long distances by undertaking seasonal migration episodes (e.g. Stutchbury et al. 2009; Wang et al. 2018). As a result, birds tend to use widely geographically separate breeding and survival habitats (sensu Alerstam and Högstedt 1982). Birds move between these areas along migration routes using staging habitats along the way, which are often shaped by the ecological barriers that separate breeding and wintering grounds (La Sorte et al. 2016) and the availability of suitable habitat patches that enable refueling after energy depletion along the way (Alerstam 1990). Long distance migratory waterbirds frequently concentrate at a narrow range of staging areas from very large breeding (e.g. Greater White-fronted Geese Anser albifrons, Wang et al. 2018) and wintering provenances (e.g. Red Knot Calidris canutus, Atkinson et al. 2005) at the continental scale. In addition, empirical observations have shown divergent migration patterns from single breeding or wintering areas via alternative routes and staging areas (migratory divides, Irwin and Irwin 2005) using ringing (Hedenstrom and Pettersson 1987) and, more recently, geolocation (Delmore et al. 2012). Many passerines are known to undertake “broad-front” migration and show little between-year fidelity to staging sites (but see Catry et al. 2011; Buler and Dawson 2014). However, first indications suggest that some wetland specialist species, whilst demonstrating large differences in patterns of site use, show striking individual between-year fidelity to staging sites, as well as to breeding and wintering areas (Hasselquist et al. 2017).

Knowledge of individual variation in migration timing and use of staging areas is critical to the understanding of migration strategies (Tøttrup et al. 2012), and the link with resources and body condition (Weber et al. 1998; Bauer et al. 2010; Armstrong et al. 2016; Thorup et al. 2017). The localization of stopover sites and their importance to flyway populations of migratory species is also essential for their effective conservation and for mitigation against the effects of land use and climate change. However, for many waterbird populations we lack such knowledge of waterbird migration routes and site use. Furthermore, we lack information to link the timing and exploitation of migration routes and staging areas, because the application of ringing (Hedenstrom and Pettersson 1987), stable isotope (Toews et al. 2017) and molecular approaches (Battey et al. 2018) fail to describe the migration of individuals in time and space (but see Delmore et al. 2012). We especially lack knowledge about how individuals differ in their speed of migration and how this relates to refueling en route. Migration behaviour is shaped to a considerable degree by a species' ability to accumulate energy stores to fuel migration episodes (Alerstam 2011), which is also affected by the range of available refueling sites. Birds may be forced to make detours and/or refuel en route if their accumulated energy stores are insufficient to undertake uninterrupted direct migratory flights (Hahn et al. 2014). For this reason, we predict that, for a species that can demonstrably accumulate fat stores to fuel a direct, nonstop autumn migration from its breeding area to the winter quarters, we would expect that the greater the number and duration of stopovers en route, the more tortuous its migratory route.

The Common Shelduck (Tadorna tadorna, hereafter Shelduck) has a widespread distribution in the Palearctic, divided into three populations, one in western Eurasia (Scott and Rose 1996) and two in East Asia (Miyabayashi and Mundkur 1999). Although these populations do not differ in morphology or plumage (Patterson 1982), they occupy very different habitats; the majority of western European birds breed and winter along shallow muddy shores and estuaries, with relatively few inland breeders. Birds breeding in semi-saline inland Mongolian waters migrate south and east to winter along Chinese and Korean coasts (del Hoyo et al. 2017). Western European birds are largely sedentary, occupying territories, which become the breeding areas, in late winter, but most adults undertake a distinctive and direct moult migration in late summer to the German Wadden Sea or to smaller moult aggregations in the UK (Wernham et al. 2002). Mongolian Shelducks moult close to their breeding areas in Mongolia (Nowak 1970) before migrating across relatively inhospitable terrain to their marine winter quarters along Yellow and China Sea coasts (e.g. Cao et al. 2010).

Almost nothing is known of the migration of this species in East Asia, which must migrate across the semi-arid biomes of Mongolia and Inner Mongolia (China) and the intensively cultivated coastal plain of Liaoning, Hebei and Tianjin provinces to reach coastal wintering habitat. The interior regions have long been subject to major cycles in precipitation (Liu et al. 2003), exacerbated in recent times by development, agriculture, irrigation and climate change, which increasingly affect the number, size and quality of potential staging wetlands along the Shelduck migratory routes (Tao et al. 2015). For this reason, it is reasonable to expect individual migrating Mongolian Shelduck to adopt differing migratory trajectories to meet energy demands en route, because of the scarce and potentially variable and unpredictable nature of the autumn wetland habitats available along the migratory route.

In this study, we tracked the autumn migration of ten Shelducks using GPS transmitters for the first time in this flyway to compare fine-scale differences in individual spatial–temporal migration patterns. We also compared within- and between-individual habitat use in post-breeding, stopover and wintering sites. We predicted that Shelducks would show diversity in space and timing of migration and corresponding diversity in habitat use, in response to the diverse refueling requirements of birds and the scarce and unpredictable nature of habitat resource availability along their migration route. In contrast, we would also predict more homogenous habitat use on the post-breeding and wintering quarters in response to more stable conditions there.

This is the first satellite-tracking study of the Common Shelduck describing the autumn migration of this widespread species, providing the first descriptions of the individual diversity in movements, timing and habitat use on autumn migration from the post-breeding grounds prior to converging on the wintering quarters along Chinese coasts. The results conformed with studies of freshwater dabbling ducks in continental western Europe that also showed flexibility in navigation and movement patterns during autumn migration (van Toor et al. 2013; Gehrold et al. 2014). Notably, the early arrival in Bohai Bay or Shandong Peninsula by two females (2173, 2220) allowed them to migrate southward to Zhejiang and Jiangsu coasts for winter. This extended movement is almost equal to the migration distance of many birds that wintered in Bohai Bay. This is in line with the broad wintering range along eastern China coasts (del Hoyo et al. 2017), and suggests that this distribution pattern is not necessarily caused by the occupancy of distinct populations (Scott and Rose 1996). It also supports the empirical observations of sex-biased wintering distribution of ducks (Owen and Dix 1986; Shao et al. 2016), which was attributed to competition (Choudhury and Black 1991). However, the seemingly sex-biased extended movement remains speculative because the difference was not significant based on t-tests because of the limited sample size. What appears unique about the results from the Shelduck tracking is the diversity of migration routes, stopover sites, stopover duration and habitat use taken by different individuals drawn from a common moulting site en route to a cluster of wintering sites. Moreover, the birds showed highly flexible migration timing. Notably, the individual 2247 migrated from the moulting areas non-stop to wintering grounds, but no other birds did so. This bird left the moulting site very late, taking 1.5 days to cover 1070 km, in a speed of 734.0 km/day. This individual was an extreme case for the timing flexibility, as all the other individuals migrated for at least 70.9 days at a speed of at most 26.0 km/day. The variability in overall migration timing would decrease if the bird 2247 was excluded. However, given the reasonably high occurrence (one in ten), we argue that the observed movements of the bird 2247 represents one type of migration strategy of this population, and legitimately contributes to the temporal variability during migration.

Many factors may explain the diverse pattern of autumn migration of Mongolian Common Shelduck. Firstly, energy store and refueling demands of birds can result in diverse site use and timing of migration. Autumn migration has been considered to be less time-constrained than in spring when birds are pressed to arrive earliest to breeding areas (Kokko 1999; Alerstam 2011). Therefore, given adequate fat stores, tracked birds were capable of making a one-step flight to complete their migration to wintering quarters, such as 2247; birds potentially failing to fully achieve such body stores might take long detours to utilize distant habitats, such as 2224, 2229 to derive fuel stores for onward movement. Secondly, the unpredictable nature of many wetlands within the reach of Shelduck migrating towards their wintering areas (Fig. 6) likely contributed to the degree of variability in site use and migration routes. Habitat unpredictability can generate high levels of variation in individual migration strategies, which are likely the product of differential search patterns and the benefit of previous experience (e.g. Roshier et al. 2008). Birds making no apparent detours used no stopover sites en route to the coast after departing the China–Mongolia border until they reached the sea, corroborated by the significant correlation between number and duration of stopover and straightness index. These differences suggest the general lack of suitable habitats in this vast area and the need to refuel to compensate for the search needed to locate suitable refueling habitats.

To better address such questions, more migratory Shelducks need to be tracked in this flyway to describe the full range of migration patterns and the reasons behind the observed differences in individual responses. We advocate modelling the effects of individual energy store accumulation at the end of the post-breeding season (adjusted for body size and sex) on subsequent migration strategy, in terms of numbers and duration of migration stopovers. Such tracking should use transmitters incorporating accelerometers, to simultaneously record instantaneous behaviour of the migrating birds. Such transmitters will help to understand the behaviour of birds at stopover sites, and provide data on energy expenditure and income during migration. Such information could contribute critical support to validate our hypothesis that these birds adopt diverse migration patterns to meet the refueling demands of a given migration episode. We would also advocate a better understanding of the habitat use of different individuals. Although the land cover dataset used here was high-resolution and new (Brovelli et al. 2015; Chen et al. 2015), it is restricted to ten crude land cover types, hindering insights into more detailed habitat selection. Ground fieldwork could verify more fine-grained habitat selection and their relative energetic profitability, which may change seasonally (Chudzinska et al. 2015). Such detailed studies would provide enlightened possibilities to compare these patterns with migration strategies of Shelduck in other parts of the world, contrast the behaviour of birds in different environments and test our hypothesis that the availability and predictability of habitats contribute directly to the diversity of migration patterns witnessed in individuals. Finally and most importantly, all the above efforts will also support the future conservation of this species, by revealing their distribution, habitat use and requirements, and potentially the impact of human activities on the birds and their refueling areas along the flyway.

We showed for the first time the autumn migration patterns of the Mongolian Common Shelduck. The results showed remarkable variability in Shelduck migration patterns, which was likely associated with refuelling patterns en route. To understand fully the migration diversity and flexibility of habitat use, we need to track more birds to increase representativeness, using accelerometer-integrated transmitters to investigate behaviours in different habitats.